Add Go 1.4 support.

This PR adds support for Go 1.4, while maintaining support for 1.3.

It also exports `UnsafeReflectValue` functionality in a new `bypass` package. This way, that functionality doesn't need to be duplicated in another package (previously [here](https://github.com/shurcooL/go/blob/34c2cb07ea1d1f14c066fc2fb7a509e0c96771a5/gists/gist6724654/main.go)) and is easier to maintain.